Conventionally, a hybrid vehicle or an electric vehicle, for example, is equipped with a battery pack, which has many battery cells connected in series, as a power supply unit, and an electric motor for driving the vehicle is driven by a voltage generated in the power supply unit.
Such a kind of power supply unit is connected to a monitoring circuit that monitors a voltage in order to prevent generation of heat, damage by fire, degradation, etc. accompanied with a voltage abnormality (refer to Japanese Patent No. 4537993, for example).
Here, between the power supply unit and the monitoring circuit is electrically connected by a plurality of detection lines for voltage detection, and there is a possibility that the monitoring circuit may be damaged when excess voltage occurs in the power supply unit or high current flows into the detection lines under influences of peripheral devices, etc.
Hence, it is necessary to provide a circuit protector that protects the monitoring circuit between the power supply unit and the monitoring circuit.
For the circuit protector, a composition that prepares circuit protection elements, such as a fuse, on each detection line, for example, may be mentioned.
Accordingly, suppression of the over-current to the monitoring circuit becomes possible because the circuit protection element of the detection line is disconnected when a large current more than a rated current flows into the detection line.
Moreover, a Zener diode (overvoltage protection element) may be connected between each detection line as a circuit protector so as to be together with the power supply unit.
Accordingly, when an excess voltage occurs in the power supply unit, a voltage applied to the monitoring circuit can be maintained at a fixed voltage because a short-circuit current flows between each detection line through the Zener diode.
By the way, the current that flows into each detection line when the excess voltage occurs in the power supply unit varies according to an internal composition (impedance) of the monitoring circuit.
Therefore, when the circuit protector is constituted from circuit protection elements such as fuses, it is difficult to set up the rated current of the circuit protection element in order that the detection line is intercepted when the excess voltage is occurring, thus the monitoring circuit cannot be protected appropriately.
On the other hand, when the circuit protector is constituted from overvoltage protection elements such as Zener diodes, the overvoltage protection element may be damaged by the large current (short-circuit current) that flows between each detection line when the excess voltage occurs in the power supply unit.
If the failure of the overvoltage protection element at this occasion is an open-circuit fault where an internal current path has been disconnected, there is a possibility that the monitoring circuit may be damaged because the excess voltage that occurred in the power supply unit is applied to the monitoring circuit.
Thus, the monitoring circuit cannot be appropriately protected from the excess voltage that occurred in the power supply unit with the conventional circuit protector, therefore, it is necessary to constitute each element in the monitoring circuit from a high voltage element with high electrical robustness, or to add protection elements in the monitoring circuit, and a complicated circuit configuration in the power supply system or cost escalation cannot be avoided.